Instead of obeying Schrödinger equation, in some materials the low-energy excitations behave as massless Dirac particles. Three-dimensional (3D) Dirac semimetals represent one such class of materials, constitutuing the closest archetype of truly relativistic massless systems. Cadmium arsenide, Cd3As2, is the prime candidate for a 3D Dirac material that is stable at ambient conditions. Two stable Dirac cones are predicted to exist around the G point of the Brillouin zone. However, a conundrum appeared regarding the scale at which the Dirac cones appear, unresolved despite a large number of experiments. Different spectroscopies claimed different outcomes; most prominently ARPES indicated a large energy spread of Dirac cones (several hundred meV) while STM/STS measurements implied that a scale at least one order of magnitude smaller.
Fig. 1: The schematic view of electronic bands in Cd3As2 with two types of conical features.
Our magneto-optical experiments lift the controversy of the electronic bands of cadmium arsenide. We show that two kinds of massless carriers can exist in this material, Dirac and Kane. In other words, the band structure includes not only one, but two types of conical features, see Fig. 1. We show that the large cone observed by ARPES is not a Dirac cone, but results in fact from the standard Kane model applied to a semiconductor with a nearly vanishing band gap. Our experiments explore the magneto-optical response of single crystalline cadmium arsenide, with two different orientations and two different positions of Fermi level. Applying a strong magnetic field, the system driven into the quatum limit. In such a case, electrons occupy only the lowest electronic Landau level and only the fundamental cyclotron resonance mode is active. Its specific character allows us to eliminate with certainty the Dirac carrier response. Moreover, no significant anisotropy is observed in the observed conical feature. These results can be quantitatively explained within the standard Kane model developed in the past for description of ordinary zinc-blende semiconductors, leading us to conclude the presence of massless Kane electrons in this material. While the symmetry-protected Dirac electrons may still be present as well, our experiments limit their range to a fairly small energy range (several tens of meV).
Magneto-Optical Signature of Massless Kane Electrons in Cd3As2, A. Akrap, M. Hakl, S. Tchoumakov, I. Crassee, J. Kuba, M. O. Goerbig, C.C. Homes, O. Caha, J. Novak, F. Teppe, W. Desrat, S. Koohpayeh, L. Wu, N. P. Armitage, A. Nateprov, E. Arushanov, Q. D. Gibson, R. J. Cava, D. van der Marel, B. A. Piot, C. Faugeras, G. Martinez, M. Potemski, and M. Orlita, Physical Review Letters 117, 136401 (2016).
The energy scale of Dirac electrons in Cd3As2, M. Hakl, S. Tchoumakov, I. Crassee, A. Akrap, B. A. Piot, C. Faugeras, G. Martinez, A. Nateprov, E. Arushanov, F. Teppe, R. Sankar, Wei-li Lee, J. Debray, O. Caha, J. Novák, M. O. Goerbig, M. Potemski, and M. Orlita, Physical Review B 97, 115206 (2018).